Fluorozirconate and fluorohafnate glasses are unique nonoxide materials which include zirconium tetrafluoride (ZrF.sub.4) or hafnium tetrafluoride (HfF.sub.4), respectively, as major constituents. These multicomponent glasses are commonly referred to as the heavy-metal fluoride glasses.
The heavy-metal fluoride glasses have been found to have certain desirable physical characteristics which make them ideally suited for a wide variety of applications in optical systems. Heavy-metal fluoride glasses are prime candidates for use as optical fibers for communications or transmission of optical power. A survey of the development of heavy-metal fluoride glasses, their properties and their uses is set forth by Martin G. Drexhage in Chapter 4 of the Treatise on Materials Science and Technology, Vol. 26 (1985), pages 151-243. The contents of this chapter are hereby incorporated by reference.
The heavy metal fluoride glasses based on the zirconium tetrafluoride-barium difluoride (BaF.sub.2) - X system (ZrF.sub.4 -BaF.sub.2 -X), where X is thorium tetrafluoride (ThF.sub.4) or one of several rare earth tri-fluorides such as LaF.sub.3 or GdF.sub.3, have been intensely studied due to desirable optical and physical properties. This new class of material was reported first by N. M. Chanthanasinh in his doctoral thesis for the University of Rennes (France) in July of 1976 and later by Poulain et al in the Materials Research Bulletin 12, 151 (1977). The vitreous domain for ZrF.sub.4 - BaF.sub.2 - ThF.sub.4 was described as an area of a triangle on the ternary diagram bounded by the following maxima: 63 mole percent ZrF.sub.4 ; 38 mole percent BaF.sub.2 and 15 mole percent ThF.sub.4.
The preparation technique typically used by these earlier workers consisted of reacting highly purified components at 800.degree. to 900.degree. C. in an argon or other inert gas atmosphere. The starting materials used in the above system were contained in a platinum crucible and the glass produced by the system was formed by pouring the fully reacted melt into a mold residing in a nitrogenfilled glove box.
A problem experienced with the above basic preparation technique and other processes involving the use of inert atmospheres is that internal bulk defects were manifested as a black-opaque secondary phase. Initially, the defects were thought to be due to reduced trace impurities present in the raw materials and thought to require exposure of the melts to air during processing. (Poulain, M. and Lucas, J. (1978); Verres, Refract, 32, 505). However, the use of air as the melt processing atmosphere has been inadvisable due to the formation of hydroxyl (OH.sup.-) and oxide (O.sup.=) impurities resulting from moisture which is present in air. These impurities absorb strongly in the mid IR portion of the spectrum, thus making the glass unsuitable for IR optic applications.
More recently, electron microprobe analysis of the secondary phase indicates that the black-opaque substance contains only one-half the fluorine content of the transparent phase and therefore is believed to be ZrF.sub.2 or ZrF.sub.3 resulting from the disassociation of ZrF.sub.4 (Robinson et al. (1980) Material Research Bulletin 15, 735). These black opaque impurities are referred to herein collectively as disassociation impurities.
A process utilizing carbon tetrachloride and hydrogen fluoride reactive atmospheric (RAP) processing to prevent formation of disassociation impurities in fluorozirconate glass melts is disclosed in U.S. Pat. No. 4,341,873. This patent is assigned to the same assignee as the present invention and the contents of this patent is incorporated by reference.
The above mentioned RAP method is not only useful in preventing disassociation impurities from forming, but is also effective in removing OH.sup.- and O.sup.= impurities. The method basically involves a final treating step in which the ZrE.sub.4 -BaF.sub.2 -X melt is treated with a reactive atmosphere or rectification mixture of He/CCl.sub.4 to rectify any color formation resulting from ZrF.sub.4 disassociation. Although this procedure has been used successfully to produce fluorozirconate glasses with enhanced optical transmission, mechanical strength and hardness; the procedure does have a few drawbacks. For example, the use of CCl.sub.4 will result in the introduction of Cl.sup.- impurities into the melt by the replacement of trace quantities of OH.sup.- with Cl.sup.- so that the anion stoichiometry is unknown. In other words, the glass fluoride stoichiometry would then be incomplete. The added Cl.sup.- also produces a higher crystallization tendency and, in extreme cases, may lead to reduced resistance to attack from environmental moisture.
It would be desirable to provide a method for treating melts of ZrF.sub.4 -BaF.sub.2 -X to prevent or reverse the formation of disassociation impurities while, at the same time, providing known and complete fluorine stoichiometry of the final melt material. By providing complete fluorine stoichiometry, the above referenced problems due to the presence of trace amounts of Cl.sup.- in the melt are eliminated.